2082 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 11, NOVEMBER 1999
Room Temperature Photonic Crystal Defect
Lasers at Near-Infrared Wavelengths in InGaAsP
O. J. Painter, Student Member, OSA, A. Husain, A. Scherer, J. D. O’Brien,
Member, OSA,
I. Kim, and P. D. Dapkus, Fellow, IEEE
Abstract— Room temperature lasing from optically pumped
single defects in a two-dimensional (2-D) photonic bandgap (PBG)
crystal is demonstrated. The high-
Q
optical microcavities are
formed by etching a triangular array of air holes into a half-
wavelength thick multiquantum-well waveguide. Defects in the
2-D photonic crystal are used to support highly localized optical
modes with volumes ranging from 2 to 3 (
/2
n
)
3
. Lithographic
tuning of the air hole radius and the lattice spacing are used to
match the cavity wavelength to the quantum-well gain peak, as
well as to increase the cavity
Q
. The defect lasers were pumped
with 10–30 ns pulses of 0.4
0
1% duty cycle. The threshold pump
power was 1.5 mW (
500
W absorbed).
Index Terms—InGaAsP, microcavities, photonic crystals, quan-
tum-well laser, semiconductor device fabrication, spontaneous
emission control.
I. INTRODUCTION
T
HE proposed use of photonic crystals to modify the
optical radiation from emitters within the crystal dates
back to the seminal papers of Yablonovitch [1] and John [2].
In this paper we report on the use of a two-dimensional (2-
D) photonic crystal to localize light to a single defect, thus
forming a high-
microcavity laser with a modal volume less
than 0.03 cubic microns. The confinement of the defect mode
energy to this tiny volume, and the predicted enhancement
of the spontaneous emission rate [3], [4] make the defect
cavity a very interesting device for low threshold lasers [5],
[6] and high modulation rate light-emitting diodes [7]. Nano-
optic structures formed from photonic crystals also hold a
great deal of promise due to the flexibility in their geometries.
Lithographic methods may be employed to alter the photonic
crystal geometry so as to tune device characteristics. An array
of densely packed photonic crystal waveguides [8], prisms [9],
and light sources [10] integrated on a single monolithic chip
may thus be envisioned. Lithographically defined photonic
crystal cavities may also find use in some material systems
as an alternative to epitaxially grown mirrors, such as for
long wavelength vertical-cavity surface-emitting lasers and
GaN-based devices.
Manuscript received July 28, 1999; revised August 27, 1999. This work
was supported by the Army Research Office under Contracts DAAH04-96-1-
0389 and DAAD19-99-1-0121, and the NSF under Contract ECS-9632937.
O. J. Painter, A. Husain, and A. Scherer are with the Department of
Electrical Engineering, California Institute of Technology, Pasadena, CA
91125 USA.
J. D. O’Brien, I. Kim, and P. D. Dapkus are with the Department of
Electrical Engineering–Electrophysics, University of Southern California, Los
Angeles, CA 90089 USA.
Publisher Item Identifier S 0733-8724(99)09084-2.
The defect cavities studied here utilize a half-wavelength
thick high-index membrane to confine the light vertically by
way of total internal reflection (TIR) similar to the design
of a whispering gallery microdisk laser. The high-index slab
is then perforated with a hexagonal array of air holes which
Bragg reflects the light in-plane. A defect is formed in the 2-
D photonic lattice by removing an air hole and/or adjusting
the diameters of a few neighboring air holes. A mode, or
set of modes depending on the defect geometry, which is
highly localized to the defect region is formed. Photons can
escape from the defect cavity by tunneling through the 2-
D photonic crystal, or by leaking out vertically from the
waveguide. An illustration of the defect cavity in cross section
is shown in Fig. 1. In earlier work [10], we performed an initial
demonstration of pulsed lasing action in defect laser cavities
where it was necessary to cool the substrate down to 150K.
The original laser cavities suffered from relatively large losses
in the vertical direction due to the large holes used to define
the photonic crystal [11]. The work presented here consists
of measurements of defect cavities in which the hole size has
been reduced, improving the quality factor (
) of the defect
modes from 250 to above 500, resulting in room temperature
pulsed operation. The paper is separated into four sections: In
Section II we describe the fabrication method, in Section III
we model the defect cavity using finite-difference time-domain
(FDTD) techniques, in Section IV we present the measured
lasing characteristics of the defect cavities, and in Section V
we provide some conclusions.
II. F
ABRICATION
The defect cavities were fabricated in the InGaAsP–InP
material system. The choice of this material system was based
upon its relatively slow surface recombination velocity [12]
which is important due to the large surface to volume ratio
present in the defect cavities. Metalorganic chemical vapor
deposition (MOCVD) was used to grow the laser structure
on an indium phosphide (InP) substrate. A schematic of
the epitaxy is shown in Fig. 2. Optical gain is provided by
four 0.85% compressively strained quaternary quantum-wells
(QW’s) [13], [14] designed for a peak emission wavelength of
1.55
m at room temperature. The barriers are also quaternary
with a room temperature bandgap of 1.22
m. Cladding
material (57.5 nm) identical to the barriers is placed on top
and bottom of the active region to isolate the QW’s further
from the surface and to increase the final waveguide thickness.
A sacrificial InP (664 nm) layer is grown beneath the QW’s
0733–8724/99$10.00 1999 IEEE
PAINTER et al.: ROOM TEMPERATURE PHOTONIC CYSTAL DEFECT LASERS 2083
Fig. 1. Illustration of a cross-section through the middle of the photonic crystal microcavity. Photons are localized to the defect region by TIR at the
air/slab interface and by Bragg reflection from the 2-D photonic crystal.
Fig. 2. InGaAsP–InP epitaxy for the defect lasers. The active region consists
of four 0.85% compressively strained InGaAsP quantum wells. The InP buffer
layer is used as a sacrificial layer which is removed by a selective HCl etch
in order to free the membrane from the substrate. The total thickness of the
membrane after processing is 211 nm.
and cladding which is subsequently removed in order to free
the membrane. A thin InGaAs etch stop layer is grown just
below the sacrificial InP layer.
Electron beam lithography is used to first define the photonic
crystal pattern in 100 nm of 2% polymethyl methacrylate
(PMMA). An Ar
ion beam etch is then used to transfer the
pattern into an Au mask layer. This is followed by a C
2
F
6
reactive ion etch which etches the holes into a SiO
2
layer. The
final dry etching step is a Cl
2
assisted ion beam etch to transfer
the air holes through the active region and into the sacrificial
InP layer. Once the dry etching is complete the remains of
the surface mask are removed using a buffered oxide etch.
The perforated waveguide is then undercut by placing the
sample in a slightly agitated HCl (4 :1) solution at room
temperature. The HCl solution enters through the etched holes
and selectively removes the underlying InP material, stopping
on the InGaAs etch stop layer, thus providing a smooth bottom
interface. Scanning electron microscope (SEM) images of the
defect cavity are shown in Figs. 3–5. The resulting membrane
is 211 nm thick and approximately 8
m across. The lattice
spacing (
) and the hole radius ( ) of the defect cavities
studied here are approximately 500 and 160 nm, respectively.
At these dimensions a forbidden frequency bandgap opens up
for the TE-like (or even) guided modes of the thin membrane
which encompasses the emission wavelength of the QW’s, as
described below.
Fig. 3. Top view of a microfabricated 2-D hexagonal array of air holes with
a single central hole missing. The interhole spacing,
a
, is 500 nm, and the
radius of the holes are approximately 160 nm.
III. DESIGN
Three-dimensional (3-D) FDTD simulations [15] can be
used to accurately model the electromagnetic modes of a
complex structure such as a photonic crystal microcavity. The
structure is discretized on a three dimensional mesh and ap-
propriate boundary conditions are applied at the outer surface
of the computational domain. In the calculations presented
here, an initial electric and magnetic field is defined on the
mesh and FDTD is used to step the field in time. The discrete
electromagnetic modes of the structure show up as resonance
peaks in the Fourier transform of the time-stepped field [16].
A given discrete mode may then be isolated in the structure by
convolving the field in time with a bandpass filter. This allows
one to associate given mode patterns and symmetries with
different resonance peaks. For localized modes of a cavity, the
power that is radiated from the computational domain may also
be calculated using the Poynting vector, from which one can
estimate the quality factor of the mode. In order to elucidate
2084 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 11, NOVEMBER 1999
Fig. 4. SEM micrograph of a defect cavity taken at an angle of 30
. After
the HCl etch the etched surfaces are smooth down to a scale from 3 to 5 nm
(SEM resolution). The sidewalls of the holes are sloped at 10
from vertical,
which can be reduced by optimizing the temperature and gas flow during the
chemically-assisted ion-beam etch.
Fig. 5. Cross section through the patterned membrane structure. The slab is
211 nm thick after processing. The use of an InGaAs etch stop layer results in
a smooth bottom interface below the membrane. For membranes larger than
10
m a significant bowing occurs and for devices greater than 15
m the
membrane collapses onto the InGaAs etch stop layer.
many of the results discussed in Section IV it will be important
to analyze the effects of varying cavity geometries on the
defect modes of the structure using these FDTD methods.
The defect microcavity studied here is formed from two
basic building blocks: A high-index slab, and a 2-D photonic
crystal. Since the high-index slab is approximately a half
wavelength thick, it is not accurate to neglect the finite nature
of such a 2-D photonic crystal. The guided modes of a
symmetric perforated waveguide can be classified simply as
even or odd modes, pertaining to the mode symmetry about the
horizontal mirror plane in the middle of the slab. The even and
odd vertically guided modes may be termed TE-like and TM-
like, respectively, in connection to the modes of a 2-D photonic
crystal which is infinite in the third direction [17]. The TE-like
and TM-like labels only provide an accurate description for
(a)
(b)
Fig. 6. In-plane band-structure of the triangular lattice of air holes in an
optically thin high-index slab for: (a) the TE-like (even) modes and (b)
the TM-like (odd) modes. The parameters used in this calculation are:
r=a
=
0
:
32
,
d=a
=0
:
409
,
n
slab
=3
:
4
, and
n
clad
=1
.
the lower lying frequency bands of the fundamental vertically
guided modes. A more detailed analysis of the modes of finite
2-D photonic crystal slabs can be found in the work by Russell
et al. [18] and Villeneuve et al. [19].
The photonic crystal lattice used in this work is a triangular
array of air holes as depicted in the SEM image of Fig. 3.
The important parameters in defining the photonic crystal
structure are the lattice spacing (
), the hole radius ( ), the slab
thickness (
), the slab index ( ), and the outer cladding
index (
). A dispersion diagram of frequency versus in-
plane momentum for the TE-like (even) modes is given in
Fig. 6(a). In this calculation
was set to 0.32, to
0.409,
to 3.4, and to 1. This corresponds to the
mean value of the parameters for the fabricated devices studied
in Section IV. The shaded region in Fig. 6(a) and (b) is for
frequencies above the light line of photons in the air cladding.
Photons with frequencies above the light line can propagate
in the air, and thus are leaky resonant modes of the slab or
PAINTER et al.: ROOM TEMPERATURE PHOTONIC CYSTAL DEFECT LASERS 2085
nonresonant radiation modes. The bands plotted below the
light line represent the guided modes of the photonic crystal
slab. As shown in Fig. 6(a) there exists a frequency bandgap
for the transverse electric (TE)-like guided modes between
the first two bands. We will use the terms “conduction” and
“valence” band to describe the upper and lower bands defining
the bandgap, respectively. The valence band is completely
guided over the entire first Brillouin zone (IBZ), while the
conduction band is leaky around the
point. The leaky nature
of the conduction band is the fundamental source of vertical
radiation loss from the defect cavity. The band diagram of the
transverse magnetic (TM)-like (odd) guided modes is plotted
in Fig. 6(b). With an
ratio of 0.32 and a slab refractive
index of 3.4 a bandgap does not form for the TM-like guided
modes. For our purposes, however, this is beneficial as it will
limit the number of high-
localized modes of the defect
(TE-like only).
Lateral confinement of the optical field is obtained by
removing a single hole in the photonic lattice creating a local
potential energy well for photons. For the index contrasts
studied here (3.4 :1) a pair of localized dipole-like modes
form which are very similar in nature to the defect modes
of the infinite 2-D photonic crystal [17]. The localized modes
of this cavity can be described as a linear combination of the
and dipole modes shown in Fig. 7. The electric field of
the diopole modes is polarized predominantly in the plane of
the slab and has an antinode in the center of the defect thus
providing good overlap with the active material. The defect
modes of this structure have been previously analyzed in detail
elsewhere [11], [19]. Using the relation [20]
(1)
the mode volume of the dipole modes is estimated to be 2.5
, which is roughly 0.03 cubic microns at a wavelength
of 1.55
m.
As the dimensions of a laser cavity are reduced, the feedback
from the laser mirrors must also increase in order to maintain
the photon lifetime in the cavity (
). For the defect cavity
which is almost a half-wavelength on a side it is critical
that the power loss from the 2-D photonic crystal mirrors is
minimized. The quality factor (
) of the localized modes in
the defect cavity can be calculated from the stored energy (
)
and radiated power (
) as follows:
(2)
where
is the angular frequency of the mode. The power
radiated from the defect can be divided into a component
due to photons which leak vertically from the waveguide, and
a component due to photons which tunnel through the finite
number of periods of the photonic crystal. The energy which
escapes by tunneling can be reduced by simply adding more
periods to the photonic crystal (limited eventually by scattering
or absorption). The power which leaks out of the waveguide
vertically can not be captured by adding more periods of the
photonic crystal, and thus poses a much more serious limitation
(a)
(b)
Fig. 7. Two-dimensional slice through the middle of the membrane showing
the electric field amplitude of the degenerate dipole modes: (a) the
x
-dipole
mode and (b) the
y
-dipole.
on the cavity . In order to reduce the vertical loss smaller air
holes are preferred, which both increases the effective index
guiding of the slab (less material removed) and reduces the
photonic bandgap (PBG). The smaller bandgap results in a
slightly shallower energy well, resulting in a mode which is
broader in real space but narrower in
-space. The narrowing
in
-space of the defect mode to regions around the band-
edge (
-point) reduces the coupling to leaky portions of the
conduction band in Fig. 6. The calculated
value for the
cavities analyzed in the next section varies between 500–600
(
and ), an improvement by a factor of
two relative to the earlier low temperature lasers. The
value
can be increased further by increasing the waveguide thickness
or by increasing the lateral dimension of the cavity [11],
[19], however this is at the expense of increasing the mode
volume and potentially increasing the number of localized
cavity modes.
IV. M
EASUREMENT RESULTS AND DISCUSSION
A large array of defect cavities was fabricated with varying
lattice spacings and hole sizes in order to cover a wide range
2086 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 11, NOVEMBER 1999
of wavelengths of the defect mode resonance. It was found
that for a lattice spacing of 525 nm and a hole radius of 165
nm the cavity mode resonance matched the gain peak of the
multiquantum well active region. The defect cavities consist
of eight periods of the photonic crystal surrounding a single
removed hole in the center. With eight periods of photonic
crystal the defect mode theoretically emits most strongly in
the vertical direction thus allowing measurements to be taken
from the top of the sample.
The sample was mounted on an X–Y–Z stage and the defect
cavities were optically pumped from above at an angle normal
to the sample surface. A high numerical aperture, long working
distance 100
objective lens was used to both image the
defect cavities and to focus the optical pump beam. An 830-nm
semiconductor laser diode was used as the pump source in this
experiment. The photoluminescence (PL) was also collected
by the objective lens and then fed into an optical spectrum
analyzer. A GaAs filter was placed just before the optical
spectrum analyzer to separate the pump from PL. The spot
size of the beam on the sample surface could be adjusted
down to 1
m, however a spot size of approximately 4 m
was found to be optimum in the sense of providing the lowest
threshold for lasing. We attribute this to the fact that the QW’s
in the photonic crystal mirror surrounding the defect will be
absorbing if not pumped to transparency. Absorption in the
2-D photonic crystal mirror will then limit the
of the defect
cavity. FDTD calculations [11] indicate that to obtain a
of roughly 200 requires at least three periods of the photonic
lattice surrounding the defect, which corresponds to a 3-
m
diameter. As the pump is defocused to increase the spot size
above 4
m the intensity in the center of the cavity decreases
to a point where the threshold pump power starts to rise. For
consistency a pump beam spot size of roughly 4
m was used
throughout to take the data.
With the pump beam focused to a 4-
m spot, we first
measured the PL from an unprocessed area on the sample very
near to the array of defect cavities. In Fig. 8 the PL spectra
is shown for various duty cycles and pump powers. At low-
pump powers (bottom plot) the emission is peaked at 1545
nm which corresponds to the first energy level in the QW’s.
As the pump power is increased (middle plot) a peak in the
emission spectrum appears at 1380 nm corresponding to the
second level in the QW’s. The top plot was obtained using
similar pumping conditions as those used to obtain lasing in
the defect cavities (10 ns pulses, 0.3% duty cycle). The PL
is very broad in this case, providing almost a 400 nm wide
emission range over which the photonic crystal cavities can
be characterized.
The PL from the defect cavities is markedly different than
from the unprocessed areas. The emission spectrum for various
pump powers from a defect cavity with a lattice spacing of 525
nm is shown in Fig. 9. The emission can be seen to be strongly
frustrated except for two peaks. The broad shorter wavelength
peak at 1425 nm most likely corresponds to spontaneous
emission from the conduction band of the photonic crystal.
The narrow longer wavelength peak at 1580 nm is the defect
mode laser line. In Fig. 10, we plot the theoretical resonances
of the defect cavity as calculated by FDTD along with the
Fig. 8. PL from an unprocessed area is shown for three different pumping
conditions (4-
m pump spot size). The bottom plot corresponds to 20
W
continuous wave (CW) pumping. In the middle plot the peak pump power is
170
W with a 50% duty cycle. The PL in the top plot was taken using 10 ns
pulses with a 3-
s period at a peak pump power of 7 mW (duty cycle
0.3%).
Fig. 9. PL spectra from a defect cavity with
a
=
525
nm and
r=a
=0
:
32
.
The pump power is increased from just below threshold (bottom) to 1.5 times
threshold (top) in the series of plots.
experimentally measured PL. The vertical dashed lines in
Fig. 10 represent the extent of the calculated TE-like guided
mode bandgap. The defect laser line is seen to match up
very well with the calculated defect mode resonance. The
spontaneous emission peak at 1425 nm (
) also
matches up very nicely with the photonic crystal conduction
band-edge. Although the modes at the conduction band-edge
are guided in an ideal slab (Fig. 6), the sidewalls supporting
the membrane (Fig. 5) may scatter the light into the collection
lens.
